A switched-mode power supply is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. A switched-mode power supply transfers power from a source, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics. The pass transistor of a switched-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy. Voltage regulation is achieved by varying the on time and the off time (i.e. duty cycle and period may be changed). A switched-mode power supply having an isolated topology includes a transformer, and thus can produce an output of higher or lower voltage than the input. Some converters use a transformer for energy storage, while others use a separate inductor.
In each case, an unexpected increase in load demand e.g. due to a short or error at the load can result in very high power draw. Protection from such over-power and short-circuit conditions is desirable.